T-cells, also known as T-lymphocytes, are a class of immune effector cells. In peripheral tissues, T-cells can be divided into two broad groups based on their mutually exclusive expression of CD4 and CD8 cell surface molecules. Typical CD8+T-cells become cytotoxic T-cells after activation and destroy antigen bearing target cells through direct cell contact. Activated CD4+T-cells generally provide positive signals, for example, "helper" function for B cells (that enable B cells to differentiate into antibody-forming cells) and, therefore, are called helper T-cells.
Six T-cell growth factors have previously been identified. The six are: Interleukin (IL) -2, -4.4, -9, -12 and cofactor IL-10. IL-2's open reading frame codes for a 15 kDa, 153-amino acid polypeptide. IL-2 is produced by certain T-cells and by large granular lymphocytes. IL-2 was originally discovered as a factor that would support long-term growth of human T-cells. In addition to T-cell growth, its effects include activation of natural killer (NK) cells and lymphokine-activated killer (LAK) cells as well as cytotoxic T-cells ("CTL"), macrophages and promotion of B-cell growth.
IL-4 is a 15-20 kDa protein produced by activated T-cells, bone marrow stromal cells, and mast cells. The IL-4 open reading frame codes for 140-amino acid murine IL-4 and 153-amino acid human IL-4. Originally, IL-4 was defined as a factor that activated B-cell growth and differentiation. Its effects also include macrophage activation and induction of class II MHC molecules, growth of some T-cell and mast cell lines, proliferation and CTL generation from human peripheral blood T-cells, enhancement of immunoglobulin production by B-cells, and a cofactor in growth of hematopoietic cells from stem cells. IL-4 plays an important role in the down-regulation of IL-2 induced NK cell and LAK cell activities. Human IL-4 is not active on murine cells.
IL-7 is a 20-25 kDa, 177 amino acid polypeptide produced by bone marrow and thymic stromal cells. Although it was originally described as a pre-B-cell growth factor, IL-7 supports the growth of pro-B-cells as well as pre-B-cells. IL-7 also induces proliferation and CTL generation from human peripheral blood T-cells, IL-2 receptor expression, IL-2 production, and proliferation in CD4.sup.+ and CD8.sup.+ cells. IL-7 also synergizes with IL-2 and increases thymic T-cell proliferation and induces proliferation of CD4.sup.- and CD8.sup.- thymocytes.
IL-9 is a 30-40 kDa, 144 amino acid polypeptide produced by activated T-lymphocytes. IL-9 was first identified as a helper T-cell growth factor. IL-9 stimulates erythroid development and enhances IL-3 induced proliferation of bone marrow-derived mast cells. It also modulates IgE and IgG production by B-cells in the presence of IL-4. Murine IL-9 is active on human cells, whereas human IL-9 does not act on murine cells.
Human IL-10 is a 16-20 kDa, 178-amino acid polypeptide produced by macrophages and TH2 but not TH1 T-helper cells. Like IL-2, IL-4 and IL-7, IL-10 has several different biological activities. IL-10 was discovered on the basis of its ability to inhibit cytokine production by activated T-cells. Both human and murine IL-10 are growth-stimulatory cofactors for thymocytes and T-cells in combination with IL-7 or IL-2 plus IL-4. IL-10 stimulates mast cell viability and growth in combination with IL-4 or IL-3 plus IL-4. IL-10 also induces the IgG secretion and expression of MHC class II molecules on B-cells and increases their viability in culture.
IL-12 is constitutive or induced by phorbol ester and calcium ionophore in lymphoblastoid cell lines and is produced by LPS stimulated macrophages. IL-12 has a molecular weight of 70 kDa and an unusual heterodimeric structure, being formed of two disulphide-bonded glycoproteins. The larger of the two glycoprotein subunits is a 40 kDa, 328-amino acid polypeptide. The smaller glycoprotein subunit is a 35 kDa, 253-amino acid polypeptide. Both glycoprotein subunits are necessary for bioactivity. IL-12 induces the proliferation of activated T-cells of both the CD4.sup.+ and CD8.sup.+ subsets independently of IL-2. IL-12 also activates NK-cell-mediated cytotoxicity and synergizes with IL-2 to generate LAK cells. Unlike IL-2 and IL-7, but similar to IL-4, IL-12 causes little or no proliferation of resting peripheral blood mononuclear cells.